This invention relates generally to cold rollforming of metal strip, and more particularly to continuous cold-rolling methods and apparatus for making metal strip which is thinned lengthwise to form laterally spaced zones of different thickness and to products produced from such strip.
Many elongated structures are formed of sheet metal which is roll-formed to a desired cross section. Examples include, but are not limited to, grid tees for suspension ceilings, metal wall studs and corrugated sheet metal. Further, in many instances, the use of the structure is such that more efficient utilization of the material forming the structure is obtained if the metal can be concentrated at specific locations within the structure.
For example, a grid tee for suspension ceilings usually provides a bulb along one edge and flanges along the opposite edges. The bulb and flanges are interconnected and maintained in a spaced relationship by a central web. In such a grid tee structure, the bulb and flanges provide the principal structural strength, and the web does not contribute very much to the strength of the structure. The main function of the web is to maintain the spacing between the bulb and the flanges. Therefore, the efficient utilization of the material of the grid tee is improved if the web thickness is reduced and the material forming the grid tee is concentrated in the grid extremities at the bulb and flanges.
United States Letters Patent No. 4,206,578, assigned to the assignee of the present invention, described a grid tee in which material is concentrated at the extremities and the advantages derived therefrom. Such patent is incorporated herein by reference.
Efficient material use is also obtained in a generally similar manner if the webs of metal studs and channels are reduced in thickness compared to the thickness of the extremities of the structure. Likewise, corrugated sheets having this connecting webs provide, in many instance, improved efficiency of material use.
Generally in the past it has been impossible or impractical to cold roll the strips of metal to provide a strip in which selected lengthwise portions have reduced thickness and other portions remain at the greater original thickness. For example, the roll-forming of the l-beam have has been performed "hot," that is, at a temperature above the recrystallization temperature, so that the material forming the beam is highly plastic before it is rolled. Other non-uniform cross section forms are also usually produced by extrusion or rolling in the hot state.
In typical cold roll-forming operation in which thickness is changed, a strip of sheet or plate material is passed between two opposed coils which apply pressure to the opposite surfaces of the material and plastically deform it to a reduced thickness. During such conventional rolling, the strip material flows primarily in a longitudinal direction, causing increased length of the strip. There is no problem involved if the reduction in thickness is to be accomplished in a uniform manner across the entire width of the strip, since the elongation tends to be uniform across the entire width of the strip.
On the other hand, if a conventional cold rolling operation were attempted to be employed to reduce the thickness of a longitudinal portion of the strip while leaving the remaining longitudinal portion at their original thickness, serious difficulties would be encountered because the reduced thickness portion would tend to expand lengthwise of the strip, while the unreduced or unthinned portions would not. This would cause the strip to curl and buckle and lose any semblance of straightness. Therefore, such an operation cannot be used to produce structures of the general type described above.
United States Letters Patent No. 4,233,833 proposes a system for forming strips of sheet metal by a roll-forming procedure so that selected lengthwise portions of the sheet are reduced in thickness, while other portions remain at the original thickness. In such patent, a method is disclosed in which a strip of material is passed over opposed corrugating rolls, while the edges of the material are laterally held a fixed distance apart. Such method purports to apply lateral tension in the material being corrugated, causing it to stretch laterally and reduce in thickness. The patent further describes the step of subsequent flattening of the corrugations. It is not believed that the method disclosed in such latter patent has ever been developed or commercially used.